I was told it was a 6520 but now I've heard it may not be. Its a upper I bought at a pawn shop with a new Doublestar lower I just got from Lone Star wholesale. The barrel has an O or a C space then MP 5.56 Nato 1/7. Its a little heavier under the handguards then the end of the barrel but not much. I guess I'd like to know what I really have. The only mark on the bolt is D, also stamped upside down under the handguard on the barrel is a 9/97